CHAPTEE VII. .
THE BASIC-BESSEMER PROCESS.
SECTION Vila.—Outline of the basic-Bessemer process.—The basic-Bessemer process consists in blowing air into liquid pig-iron for the purpose of burning most of the silicon, manganese, carbon, phosphorus and sulphur of the metal, the operation being conducted in a basic-lined vessel, and in such a manner that the product is entirely fluid. The method by which the air is introduced has little effect on the product, but the use of a rotary vessel with bottom blast is universal.
The distinctive feature of the basic vessel is a lining which resists the action of basic slags; this is1 almost always made of dolomite. The stone must be burned thoroughly to expel the last traces of volatile matter and then ground and mixed with anhydrous tar. The bottom is generally made by ramming the same material around pins which are withdrawn after firing. At one German works magnesite tuyeres are used which last seventy heats, but the cost is high and the practice has not been generally adopted.
The highest function of the lining is to remain unaffected and allow the basic additions to do their work alone, so that the rapid destruction of a basic, as compared with an acid lining, is not due to any necessary part it plays in the operation, but to the fact that there is no basic material in nature which, by moderate heating^ will give the firm bond that makes clay so valuable in acid practice.. The agent used in its place is a rich tar, and this forms a coke under the action of heat and resists the scouring of metal and slag, and, by the time this coke is burned, the dolomite has become partially fused and "set." There is always, however, a slight shrinkage in the burned stone, no matter how thoroughly it has been roasted, so that there is a tendency to self-destruction through the formation of innumerable disintegrating cracks.
"When air is blown through pig-iron, the first element'affected is